Reverse Microemulsion Synthesis, Structure, and ...sites.ffclrp.usp.br/biotr/artigos/Osa24.pdf ·...

Reverse Microemulsion Synthesis, Structure, and Luminescence of Nanosized REPO4:Ln3+

(RE ) La, Y, Gd, or Yb, and Ln ) Eu, Tm, or Er)

Paulo C. de Sousa Filho and Osvaldo A. Serra*Departamento de Quımica, Faculdade de Filosofia, Ciencias e Letras de Ribeirao Preto, UniVersidade de SaoPaulo, Ribeirao Preto, SP, Brazil

ReceiVed: October 18, 2010; ReVised Manuscript ReceiVed: NoVember 22, 2010

A surfactant-mediated solution route for the obtainment of nanosized rare-earth orthophosphates of differentcompositions (LaPO4:Eu3+, (Y,Gd)PO4:Eu3+, LaPO4:Tm3+, YPO4:Tm3+, and YbPO4:Er3+) is presented, andthe implications of the morphology control on the solids properties are discussed. The solids are prepared inwater-in-heptane microemulsions, using cetyltrimethylammonium bromide and 1-butanol as the surfactantand cosurfactant; the alteration of the starting microemulsion composition allows the obtainment of ∼30 nmthick nanorods with variable length. The morphology and the structure of the solids were evaluated throughscanning electron microscopy and through powder X-ray diffractometry; dynamic light scattering and thermalanalyses were also performed. The obtained materials were also characterized through vibrational (FTIR)and luminescence spectroscopy (emission/excitation, luminescence lifetimes, chromaticity, and quantumefficiency), where the red, blue, and upconversion emissions of the prepared phosphors were evaluated.

1. Introduction

The use of rare-earth (RE)-based phosphors began aroundthe 1970s, aiming at the generation of the three primary colorsin cathode ray tubes and fluorescent lamps. Since then, theevolution and diversification of RE-bearing inorganic phosphorshave followed the development of new visualization/lightingtechnologies, in the sense that they must be compatible in termsof stability and efficiency with the excitation source and theworking method of each particular system.1-5 The great interestin trivalent lanthanoid cations arises from their unique intrac-onfigurational f-f transitions, which can occur as sharp andintense emission lines. For example, the Eu3+ red and the Tb3+

green emissions are extensively employed in many luminescentdevices, besides finding many analytical applications.3 Inaddition, due to its chemical stability and high emission colorpurity, and although it normally has very low emission intensi-ties, Tm3+ remains a very promising candidate for generationof the blue color in such systems, to substitute for the unstableEu2+ ions that are currently used.3,4,6

Within the wide range of applications of RE-containingphosphors, the rare-earth orthophosphates (REPO4) have aprominent role in most of these technological fields, becauseof their physicochemical inertia (stability under high tempera-tures, high-energy excitations, and chemical decomposition).7-9

For example, these compounds are very suitable phosphors forvacuum ultraviolet (VUV; λexc < 200 nm) applications, such asplasma display panels and Hg-free fluorescent lamps. In thesecases, their applicability arises from their excitability in the VUVrange, high visible light reflectivity, and resistance underoperational conditions and against annealings for deposition.4-9

Moreover, besides conventional linear processes, REPO4 phos-phors can also efficiently present near-infrared (NIR) upcon-version luminescence, thus being applicable as NIR detectors,lasers, and imaging agents.10-12 In particular, the expected lowtoxicity and the photochemical stability of luminescent or

upconverting RE-bearing phosphates allow their use as biolabelsin noninvasive medical diagnostic procedures after a properbiofunctionalization process.13-15 In most of these examples,the morphology of the solids is a limiting factor for theirusefulness, and high surface-to-volume ratios are required forgood efficiencies to be achieved. Under VUV excitation, forinstance, the low penetrability of this radiation implies that mostof the luminescence phenomena occur on the surface atoms, sothat nanosized solids are preferred.4,5 On the other hand, particlesize distribution must be narrow and with a mean value that iscompatible with the expected biological role in the case ofbiolabeling applications, so that an efficient targeting is at-tained.15

In summary, all of these applications are limited by a delicatebalance among morphological, compositional (purity, activator/sensitizer concentrations, and stability), and structural-spectro-scopic (appropriate site symmetry for activator ions) properties.Thus, to attain control of these properties at the molecular level,efficient synthetic procedures that are clean and inexpensive andthat consume low energy are required. Because routes involvingreactions in solution are preferred to fulfill these requirements,much attention is currently being paid to wet-chemical methodsfor the production of nanosized REPO4, such as colloidalsyntheses16 and solvothermal17-19 and hydrothermal20-22 pre-cipitations. In this sense, surfactant-mediated processes,9,11,23,24

mainly those based on reverse microelumsions, are also beingwidely explored since they allow a bottom-up control of themolecular organization of the nanostructure design, the so-called“zeptoliter chemistry”.

Therefore, this work deals with the establishment of asynthetic route for the preparation of REPO4 in water-in-oilmicroemulsions, with the evaluation of the effect of the startingcomposition of the microemulsions on the morphology of thematerials. It has been possible to demonstrate that REPO4 ofcontrollable morphology and with several compositions can beobtained via a simple procedure carried out in solution, showingthat highly stable phosphors with different luminescent behaviorcan be prepared for a wide range of applications.

* To whom correspondence should be addressed. Phone: +55-16-3602-3746or +55-16-3602-4376. E-mail: [email protected].

J. Phys. Chem. C 2011, 115, 636–646636

10.1021/jp109988a 2011 American Chemical SocietyPublished on Web 12/15/2010

2. Experimental Section

2.1. Synthesis. Rare-earth nitrate stock solutions (1.0 molL-1) were prepared through the dissolution of the previouslycalcined rare-earth oxides (La2O3, Eu2O3, Er2O3, Tm2O3, Yb2O3,99.99%, Rhone-Poulenc/Rhodia; Gd2O3, 99.99%, Strem-Chemicals; Y2O3, 99.99%, Alfa-Aesar) in concentrated nitricacid. The pH of these solutions was adjusted to ∼4 throughevaporation of the acid in excess. In the synthetic procedures,the RE(NO3)3 solutions were properly mixed so that thefollowing compositions would be attained (1.0 mol L-1 inRE3+): La0.99Eu0.01, Y0.64Gd0.35Eu0.01, La0.99Tm0.01, Y0.99,Tm0.01,and Yb0.95Er0.05. For further evaluation of the effect of acomplexing agent on the morphology of the solids,(La0.99Eu0.01)(NO3)3 solutions containing 1.0 mol L-1 citric acid(H3cit) were also prepared and used in the syntheses. Theprocedure employed for the preparation of the nanosizedpowders was the same in all cases, and the orthophosphatesource was always NaH2PO4 ·H2O (99+%, Aldrich). First, thetwo starting water-in-oil microemulsions were prepared. Theutilized organic phase was n-heptane (99.5%, J.T. Baker), andthe water:oil volume ratio was kept at 1:10 in all the prepara-tions. Cetyltrimethylammonium bromide (CTAB; 98%, Vetec,previously recrystallized twice from ethanol) and 1-butanol(99.5%, Merck) were used as the surfactant and cosurfactant,respectively (at a fixed CTAB:1-butanol mass ratio of 70%).The microemulsions, which were limpid and monophasic in allcases, were prepared by dispersing the appropriate volumes ofrare-earth nitrate or NaH2PO4 (0.55 mol L-1) aqueous solutionsin n-heptane (so that a 10% PO4

3- excess would be obtained).Different water:CTAB (ω0) molar ratios (ω0 ) 15, 30, 40, or50) were employed. The microemulsions were aged for 24 hbefore any measurement and before being used. The reactionswere initiated by adding the RE(NO3)3-containing microemul-sion dropwise to the microemulsion-containing NaH2PO4 undervigorous stirring, and the system was then kept under refluxfor 24 h to attain high yields and high particle homogeneity. Itis worth mentioning that both microemulsions must have thesame ω0 ratio. Finally, the products were separated fromthe resulting monophasic microemulsions by centrifugation. Thesolids were washed with water until the washing tested negativefor bromide ions (as detected with saturated AgNO3), whichwas followed by washing with an additional 50 mL of waterand then 25 mL of ethanol. The solids were dried in vacuum ina desiccator over silica (yield 70-85%). In all cases, hydratedsolids were obtained. For elimination of the hydration waters,the solids were treated with triethyl orthoformate (TEOF;Aldrich, 99%) in anhydrous EtOH (∼100 mg of solid/10 mLof EtOH/5 mL of TEOF) for 4 h under reflux. After separationby centrifugation and washing with anhydrous EtOH, the solidswere annealed at 400 °C for 24 h in air. For further comparisonand mainly for the luminescence measurements, the solids werealso postannealed at 900 °C for 2 h, thus resulting in hexagonal-to-monoclinic or hexagonal-to-tetragonal structural conversionsin the phosphates.

2.2. Characterization. Morphological analysis of the ob-tained powders was accomplished with a Jeol JSM 7401F fieldemission microscope (SEM-FEG) and a Zeiss EVO50 scanningelectron microscope coupled to an IXRF Systems 500 digitalprocessing accessory (SEM-energy-dispersive system, EDS).The structural analysis was performed by powder X-ray dif-fractometry (XRD) in a Siemens D5005 diffractometer, usingthe Cu KR radiation (1.541 Å) and a graphite monochromator.The thermal behavior of the solids was evaluated in a TAInstruments SDT simultaneous differential thermal analysis-

thermogravimetric analysis (DTA-TGA) Thermal Analyst2100, operating under a synthetic air atmosphere (100 mL min-1

flux) at a heating rate of 10 °C min-1. The vibrationalspectroscopy data of the materials were obtained by Fouriertransform infrared (FTIR) spectroscopy using KBr pellets inan ABB Bomen FTLA2000-100 spectrometer with 2 cm-1

resolution.The hydrodynamic data of the microemulsions and solids

were acquired by dynamic light scattering at 298 K in a MalvernInstruments Zetasizer 3000 HSA photon-correlation spectrom-eter. For particle size determination in the microemulsions(Supporting Information), the viscosity and the refractive indexof pure n-heptane were employed; for the �-potential determi-nation, ∼10-4 mol L-1 suspensions of the solids were preparedusing different solutions as dispersing media (0.01 mol L-1

acetic acid/acetate buffer for pH 2 and 4, 0.01 mol L-1

ammonium acetate for pH 7, and 0.01 mol L-1 ammonia/ammonium buffer for pH 10 and 12). The luminescence spectraand lifetime measurements (with direct excitation on the solids)were recorded on a Horiba-Jobin Yvon SPEX Triax 550FluoroLog 3 spectrofluorometer equipped with a continuous 450W xenon arc lamp and a pulsed 150 W xenon lamp as sources,besides a Hamamatsu R928P photomultiplier and a Peltier-cooled SPEX Synapse charge-coupled device (CCD) as detec-tors. The emission and excitation spectra were corrected withrelation to the detector sensitivity and lamp intensity in themonitored wavelength range by using the apparatus software.

3. Results and Discussion

3.1. Morphological and Structural Characterization. Fig-ures 1-3 display the SEM-FEG micrographs of the LaPO4:Eu3+ powders obtained at different ω0 ratios (15, 30, 40, 50).The results clearly show that the proposed method is adequatefor morphological control of the REPO4 synthesis at thenanoscale. As seen in Figure 1, the prepared powders arecomposed by microaggregates of elongated nanostructures ofvariable length, which are dependent on the starting ω0 ratio.Nevertheless, in all cases, a high thickness uniformity of 20-30nm is observed, regardless of the water:surfactant ratio.

At higher surfactant concentrations (ω0 ) 15-30, Figure1a,b), quite similar nanorods of 50-100 nm length are obtained.They occur as micrometric aggregates of ∼1 µm length and250-500 nm thickness. In situations of lower surfactant content(ω0 ) 40, Figure 1c), a distinct microemulsion molecularorganization may lead to the formation of 30 nm thick nanowiresof expressive lengths (>1 µm). This reflects a possible lamellarorganization in this microemulsion system, in which the waterpools may be confined with a higher degree of continuity, whichwould give rise to quite elongated REPO4 nanostructures. Onthe other hand, the solids obtained at ω0 ) 50 (Figure 1d)present a lower degree of particle agglomeration, being com-posed by well-defined nanorods of 100-200 nm length.

The observed elongated structures suggest that the rate ofinorganic nucleation is higher than the rate of micellar fusion/dissociation, so that an approximately cylindrical particle isexpected for each micellar collision. Therefore, further micellardynamic exchange may favor faster subsequent growth at theends of the initial particle, which should yield the elongatedstructures that later aggregate and segregate from the micro-emulsion (Figure S1, Supporting Information).25 The fact thatthe particle thickness is independent of the ω0 ratio indicatesthat the nucleation and the formation of the nanorod seeds arevery similar in all cases; further differentiation (affected by themicroemulsion composition) takes place in the subsequentprocesses of particle growth.

Microemulsion Synthesis of (RE)PO4 J. Phys. Chem. C, Vol. 115, No. 3, 2011 637

To evaluate the evolution of particle morphology along thereaction, aliquots of ∼1 mL of the reaction mixtures werecollected at different times. The SEM-FEG images of the solids[obtained at time 0 (completion of the mixture of the startingmicroemulsions) and 4, 8, and 24 h of reaction for mixtureswith ω0 ) 40 and ω0 ) 50] after separation by centrifugationare shown in Figure 2. The micrographs illustrate the lack ofinfluence of the ω0 ratio on particle nucleation and formationof nanorod seeds since, at t ) 0, practically identical particlesare observed in the two monitored cases. Moreover, theoccurrence of particle growth at the ends of the nanorod seedsis also evidenced by the micrographs.

On the basis of the observed behavior, the addition of achelating agent to the aqueous precursors should diminish therate of inorganic nucleation, thus retarding the growth at the

ends of the nanoparticles. Figure 3 depicts the SEM-FEGmicrographs of the solids obtained using RE(NO3)3 solutionscontaining 1.0 mol L-1 H3cit. The images show that, as expected,the complexing action of the citric acid species in solutionenables nanorod stabilization by avoiding subsequent growth.Moreover, the particles present a lower degree of aggregationafter the synthesis, which is also an effect of the action of H3cit.Again, the thickness of the particles remains practically thesame, while the length of the nanorods can be controlled bythe starting ω0 ratio (ω0 ) 15, 30-50 nm; ω0 ) 30, 70-90nm; ω0 ) 40, 100-150 nm; ω0 ) 50, 70-90 nm).

The stability of the different hydrated LaPO4:Eu3+ nanopar-ticles in suspension was electrokinectically assessed by deter-mination of the �-potential, which allows for estimation of theelectrostatic repulsion of the particles. To this end, suspensions

Figure 1. SEM-FEG micrographs of LaPO4:Eu3+ obtained in the absence of H3cit at different water:surfactant starting ratios: (a) ω0 ) 15, (b)ω0 ) 30, (c) ω0 ) 40, (d) ω0 ) 50.

Figure 2. Evaluation of the reaction time effect on the LaPO4:Eu3+ particle size and morphology in a synthesis carried out with (a) ω0 ) 50 and(b) ω0 ) 40 in the absence of H3cit.

638 J. Phys. Chem. C, Vol. 115, No. 3, 2011 de Sousa Filho and Serra

of the prepared powders were submitted to electrophoreticmobility measurements by dynamic light scattering. Figure 4ademonstrates that the particles obtained in the absence of H3citpresent the same behavior with regard to pH, showing moderatecolloidal stability under extreme acid or basic conditions (30 <|�| < 50 mV). Moreover, a slight change in the isoelectric point(i.e., pH at which � ) 0) is observed for the different ω0 ratios(pHiep follows the order ω0 ) 15 < ω0 ) 30 < ω0 ) 40 ≈ ω0

) 50), which can be related to the morphological differencesand, thus, to the area available for the surface charges. The samebehavior is observed for the pHiep values of the solids obtainedin the presence of H3cit, but the mean isoelectric point value islower in the latter cases. The profile of the plot of �-potentialagainst pH is also different for the latter solids, with two maximaat pH 4 and 10. This can be attributed to the presence of a smallamount of residual citric acid species on the surface of theparticles, which affects the overall surface charge because oftheir acid-base equilibria.

The thermal behavior of the synthesized powders was alsoevaluated by TGA-DTA analysis, which showed that the solidspresent almost the same behavior (independent of the startingω0 ratio). As seen from the thermal analysis curves (SupportingInformation), the decomposition of the nanopowders occurs intwo endothermic steps. The first one appears in the temperaturerange between 50 and 150 °C and is associated with the loss ofphysisorbed water and surface hydration waters. The seconddecomposition step, centered at 200 °C, corresponds to the lossof “zeolitic” water molecules (i.e., those occupying definedcrystallographic sites at the channels present in the rhabdophanestructure),26 as well as the beginning of conversion from thehexagonal to monoclinic phase. The DTA curves reveal thatthe loss of water molecules is followed by the endothermicstructural conversion, which is completed between 600 and 800°C. As shown in Tables S2 and S3 (Supporting Information),the composition of the solids can be estimated by consideringthese two steps of water loss. The results indicate that all solidsdisplay the same degree of structural hydration (zeolitic waters)

since they present the same percentage mass loss in the secondstep, which in turn results in the (La,Eu)PO4 ·0.6H2O ap-proximate composition. The overall composition, obtained by

Figure 3. SEM-FEG micrographs of LaPO4:Eu3+ obtained with the addition of H3cit to the RE(NO3)3-containing microemulsions: (a) ω0 ) 15,(b) ω0 ) 30, (c) ω0 ) 40, (d) ω0 ) 50.

Figure 4. �-potential as a function of pH for the hydrated LaPO4:Eu3+ nanoparticles synthesized (a) in the absence of H3cit and (b) withaddition of H3cit.


also taking the surface waters into account, is virtually the samefor the ω0 ) 15, 30, and 50 (obtained in the absence of H3cit)preparations [(La,Eu)PO4 ·1.4H2O], which is in agreement withtheir similar morphological properties. On the other hand, thesolid obtained at ω0 ) 40 exhibits a higher degree of phys-isorbed water, namely, a (La,Eu)PO4 ·3.3H2O overall composi-tion, associated with the occurrence of nanowires instead ofnanorods. The solids synthesized from H3cit-containing micro-emulsions display a very similar behavior, with the same twodecomposition steps, besides an additional exothermic stepcentered at ∼300 °C, which is related to the oxidation of theadsorbed H3cit. In the latter case, the compositional calculationsyield a similar value of structural hydration (around 0.6), besidesa similar overall water content (of about 1.1-1.3), comparedto those of the previous samples.

The powder X-ray diffractograms attest that the proposedmicroemulsion synthesis leads to the production of hydratedphosphates. In the case of the LaPO4:Ln3+ samples, the solidspresent the rhabdophane-type (REPO4 · xH2O; RE ) La, Ce, Nd;0.5 < x < 1) primitive hexagonal structure, whose spatial groupsare P6222 (D6

4, No. 180) and P3121 (D34, No. 152).26 Figure 5a

corresponds to the XRD patterns of the hydrated LaPO4:Eu3+

samples. In all the cases, the occurrence of largely broadenedpeaks indicates that the solids are composed by nanostructures.Nevertheless, because the particles are not spherical, theapplication of the usual Debye-Scherrer method (with K )0.9) for estimation of the crystallite size was not performed dueto lack of a proper shape parameter. Moreover, no peaks ofother phases or impurities were detected, thus attesting to thepurity of the solids. The observed diffraction profiles slightlydiffer from the rhabdophane pattern with regard to the relativeintensities of some peaks, which indicates the occurrence ofanisotropic growth during the formation of the crystallites. Inthis case, for example, crystal growth in the [010] plane (2θ ≈14.5°) is defavored with regard to that in the [011] (2θ ≈ 20°)and [100] (2θ ≈ 25°) planes. This is related to the structure-directing action of the reverse-micelle aggregates, which, asalready discussed, favor the growth processes at the ends ofthe nanoparticle seeds in the micellar collisions, thus resultingin elongated structures. Figure 5b illustrates that the TEOFtreatment for elimination of hydration waters is mild, so it doesnot significantly affect the structures of the solids since thehexagonal rhabdophane phase is still dominant. Nevertheless,some additional peaks, which are related to the presence of themonazite phase, are observed, giving evidence that a smallamount of the hexagonal phase is collapsed, thereby yieldingthe monoclinic phase. As shown in Figure 5c, the solids assumethe monoclinic monazite structure (of P21/m (C2h

2) space group,No. 11) after the calcinations at 900 °C, again without peaks ofphases due to contaminants. The diffractograms also presentconsiderable peak broadening, indicating that the solids maintaina nanostructural character even after the calcinations. Moreover,the crystallite anisotropic growth is also evidenced by the largeintensification of the [012] reflection (at 2θ ≈ 30.9°) comparedwith the [-120] (2θ ≈ 28.9°) and [200] (2θ ≈ 26.8°) reflections.The LaPO4:Eu3+ solids synthesized in the presence of H3citpresent very similar diffraction behavior (Supporting Informa-tion), with the same anisotropic growths before and after thecalcinations, besides large peak broadening.

The powder X-ray diffractograms of the other synthesizedphosphates are presented in the Supporting Information (FiguresS9-S13). As expected, the LaPO4:Tm3+ phosphor has propertiesvery similar to those of the Eu3+-doped lanthanum phosphates.The samples containing yttrium or ytterbium as the main host

components [(Y,Gd)PO4:Eu3+, YPO4:Tm3+, and YbPO4:Er3+]are also obtained in their hydrated forms, with a rhabdophane-type P6222/P3121 structure, thus being isostructural to thehexagonal YPO4 ·0.8H2O (JCPDS 420082). In these cases, nocontaminations of the more stable body-centered tetragonalxenotime-type phase (of I41/amd (D4h

19), No. 141, space group)are observed in the hydrated or TEOF-treated solids, thusattesting to the high selectivity of the microemulsion synthesiswith respect to the obtainment of hexagonal phosphates. Afterthe calcinations, these phosphates assume the tetragonal xeno-time structure. The calcined YPO4:Tm3+ and YbPO4:Er3+

samples present diffraction patterns which are coherent with

Figure 5. Powder X-ray diffractograms of the LaPO4:Eu3+ samplessynthesized in the absence of H3cit: (a) hydrated, (b) TEOF-treated,(c) calcined.


the xenotime standard. On the other hand, because of the highconcentration of Gd3+ ions (which tend to form a monoclinicGdPO4 phase), the relative intensity of some reflections isstrongly changed in the case of the (Y,Gd)PO4:Eu3+ sample.This indicates the occurrence of very prominent preferentialgrowths, which is in agreement with what has been observedfor similar preparations.8 Nevertheless, good incorporation ofthe Gd3+ ions into the YPO4 host is confirmed by the absenceof reflection peaks that are not related to the tetragonal phase.

3.2. Vibrational Spectroscopy. The infrared spectra of theprepared phosphates (Supporting Information) evidence thepurity of the obtained compounds since bands related tohydrogen phosphates, which can lead to contaminations withpolyphosphates after the thermal treatment, at ∼900 cm-1

(P-OH stretching) are absent. Moreover, the efficiency of thewashing processes is confirmed by the absence of bands ascribedto the surfactant molecules, such as C-H stretching (at ∼2900cm-1) and CH2 scissoring (at ∼1450 cm-1) vibrations. In theLaPO4:Eu3+ samples prepared in the presence of H3cit, ad-ditional bands of low intensity at 2925 and 2853 cm-1 and at1412 cm-1 are observed, which are related to the presence ofsmall amounts of citrate species on the particle surface.22 Inthe hydrated LaPO4:Ln3+ samples, a characteristic profile oforthophosphate groups under C2 symmetry is observed, whichis in agreement with the space group of the solids in thehexagonal structure.26,27 In these cases, three groups of absorp-tion bands are observed, one of low intensity related to the P-Osymmetric stretching (ν1, A) at 950 cm-1, one broad and intenseband composed by at least three components related to P-Oantisymmetric stretching (ν3, A + 2B) centered at 1050 cm-1,and three bands ascribed to the O-P-O antisymmetric defor-mation (ν4, A + 2B) at 613, 570, and 542 cm-1. As yttriumphosphate-based samples were also obtained as rhabdophane-type hydrated solids, in these samples the orthophosphate ionsalso occupy C2 sites, giving rise to a band unfolding similar tothe previously described ones. As already mentioned, for thehexagonal LaPO4 phosphors, the TEOF treatment leads to partialconversion of the hexagonal phase into the monoclinic one. Thisis evidenced by the appearance of an additional component inthe ν4 bands, besides the intensification of the ν1 stretching band.The same occurs for the hexagonal yttrium phosphate-basedsamples. After the thermal treatment at 900 °C, the hexagonalLaPO4:Ln3+ samples are completely converted into monoclinicphosphates. In these cases, the PO4

3- groups occupy Cs or C1

sites; under such symmetries, all the combinations of funda-mental vibrations are IR-allowed, leading to high band unfold-ings. In these samples, the P-O symmetric stretching band (ν1)occurs at 952 cm-1, the P-O antisymmetric stretching band(ν3) presents five observable components at 1120 (shoulder),1090, 1060, 1015, and 990 cm-1, and the antisymmetric bending(ν4) appears as a doublet pair with frequencies at 613, 578, 565,and 542 cm-1. On the other hand, the YPO4:Ln3+ and YbPO4:Er3+ samples are converted into tetragonal phosphates after thecalcinations; in these cases, the PO4

3- groups occur as distortedtetrahedra with D2d symmetry. In such cases, the orthophosphateions have only two IR-active vibrations, one ascribed to the ν3

stretching (B2 + E) at ∼1100 and ∼1025 cm-1 and anotherascribed to the ν4 deformation (B2 + E) at ∼635 and ∼530cm-1. In the calcined (Y,Gd)PO4:Eu3+ sample, the presence ofadditional bands in the ν4 region denotes the coexistence ofanother phosphate phase, which can be ascribed, as furtherdiscussed, to a small amount of monoclinic GdPO4 phase.

3.3. Luminescence Spectroscopy. 3.3.1. Eu3+-Doped Phos-phates. The emission and excitation spectra of the LaPO4:Eu3+

samples obtained at different conditions are shown in Figures6-9. The as-prepared (hydrated) solids present very lowluminescence as a consequence of the presence of a largenumber of quenching species (OH groups).3,4 On the other hand,after the TEOF treatment and drying, the solids start to displaymore significant emissions. The excitation spectra of the TEOF-treated LaPO4:Eu3+ samples (Figure 6a) are, for all ω0 ratiosand regardless of the presence of H3cit, composed by a broadband which appears as a shoulder at 265 nm, assigned to theO2- f Eu3+ charge transfer band (CTB), besides the charac-teristic f-f Eu3+ absorptions, the 5L6 r

7F0 transition at 394nm being the most intense line. There is no expressive shift inthe CTB position with regard to the different ω0 ratios, whichmeans that the solid band structure was not significantly affectedby the morphological alterations. In these cases, the emissionspectra of the solids (Figure 7a) under CTB or 5L6 excitationare composed by a set of broad 5D0 f

7FJ bands with a largeunfolding, which indicates the occupation of low-symmetrysites.3,4,28 This is due to the coexistence of the hexagonal andmonoclinic phases in the TEOF-treated phosphates, besides thelow degree of crystallinity of the samples, which together resultin the occupation of multiple sites with similar properties. Thisis also clearly indicated by the existence of two bands attributedto the 5D0f

7F0 transition at 577.0 and 577.5 nm. Besides, theoccurrence of broadened bands leads to chromaticity coordi-nates29 (Table 1) which are relatively far from the ones requiredfor a red phosphor.4

In the excitation spectra of the calcined solids (Figure 6b), astrong intensification of the CTB at 265 nm compared with thef-f absorptions is observed as a consequence of the highercrystallinity of these samples. Once more, the solids obtainedwith different ω0 ratios and in the presence or absence of H3citpossess essentially the same emission/excitation profile. Besides,no alterations in the position or in the relative intensities of thef-f absorptions are noticed, thus revealing large site similaritybefore and after the calcinations. Moreover, the occupation ofsites with high centrosymmetric character is also evidenced bythe low intensity of the 5D2 r

7F0 transition at 464 nm, which

Figure 6. Excitation spectra (λem ) 616 nm) of the LaPO4:Eu3+

samples (a) after the TEOF treatment and (b) after calcination at 900°C (in this case, obtained at ω0 ) 50). Insets: excitation spectra withoutthe lamp intensity correction.


takes place through forced electric dipole and vibronic couplingmechanisms and is also hypersensitive to the site symmetry withregard to an inversion center (∆J ) 2).3,28 The emission spectraof the solids (Figure 7b) are composed by the 5D0 r

7FJ lineswith the maximum 2J + 1 unfolding for a single site, whichattests to the occupation of a low-symmetry site (C1, Cs, Ci)and is in agreement with the sites available in the P21/mmonoclinic structure. In this case, the 5D0f

7F0 transition occursas a sharp line (∼10 cm-1 width) at 17316 cm-1, therebyindicating that the postannealing results in the monoclinic phaseonly, with a single symmetry site for the Eu3+ ions. Despitethe predominance of the orange 5D0 f

7F1 emissions over the5D0 f

7F2 ones, the sharpening of the emission bands after thecalcinations and the high intensity of the 5D0 f

7F4 transition(in the deep red region) lead to very high emission red colorpurities (x ≈ 0.67, y ≈ 0.33, Table 1) which are exactly the redNTSC standards.4

The luminescence lifetimes of the synthesized solids werealso investigated and are listed in Table 1. The decay curvesare presented in the Supporting Information. The TEOF-treatedsamples display a biexponential behavior as a result of thecoexistence of the hexagonal and monoclinic phases, thus givingrise to at least two emitting centers. The deconvolution of theexperimental decay curves yielded lifetimes of τ1 ≈ 3.9 and τ1

≈ 0.8 ms, approximately, for all the cases, regardless of thecomposition of the starting microemulsion. In spite of beingrelatively high (in agreement with the elimination of thehydration waters), these luminescence lifetimes are quite lowcompared with those observed after the calcinations (Table 1),which clearly display a monoexponential excited-state decaywith lifetimes around 5.0 ms. This is indeed expected, becausethe low crystallinity of the samples in the first cases providesmany nonradiative means of excited-state deactivation. Never-theless, the lifetimes observed for the calcined LaPO4:Eu3+

samples are quite high, which is probably related to both thehigh crystallinity of the samples and their nanostructuralcharacter. The high crystallinity guarantees a high atomicorganization, which provides efficient activator separation (todiminish the rate of concentration quenching) and a very lowinteraction of the emitting states with the lattice phonons. Onthe other hand, the nanostructural character leads to higher

surface areas and, consequently, to larger amounts of surfacedefects, which can act as traps for the luminescent centers byinteracting with the emitting states and increasing their lumi-nescence lifetimes.30

Figure 8 shows that the (Y,Gd)PO4:Eu3+ excitation spectradisplay, besides the characteristic Eu3+ lines, very intense linesascribed to Gd3+ absorptions (mainly the 6IJ r

8S7/2 and 6PJ r8S7/2 transitions at 271 and 303-310 nm) which dominate thespectra. This evidences an efficient sensitizing action of the Gd3+

ions in this compound, thus enabling the exploration of “photon-cascade” or “downconversion” processes (quantum yieldsgreater than unity) with this red phosphor under proper excitationat ∼200 nm.4 In this sample, the f-f lanthanoid absorptionsare also accompanied by an intense charge transfer bandcentered at 230 nm. The occurrence of a CTB at higher energiescompared with those observed in the LaPO4:Eu3+ samples arerelated to the stronger ionic character of the YPO4 host, whichleads to a low-lying valence band (VB). Consequently, theseparation between the VB and the divalent state of theeuropium ions (f7) is increased, leading to a CTB at higherenergies.31

In the emission spectrum of the TEOF-treated (Y,Gd)PO4:Eu3+ sample (Figure 9a), a large predominance of the 5D0 f7F1 transition over the 5D0f

7F2 transition indicates the possibleoccupation of sites with a high centrosymmetric character, whichis in agreement with the occupation of D2 sites in the hexagonalstructure. This high intensity of orange emissions does notcompromise the purity of the red color emission of thispreparation (x ≈ 0.67 and y ≈ 0.33). In this solid, thecoexistence of at least two cation sites is evidenced by theoccurrence of a very broad (∼90 cm-1 width) 5D0 f

7F0

transition, which can be deconvoluted into at least twocomponents (Figure 9a, inset). This is a consequence of thecoexistence of D2 and D2d sites of the hexagonal and tetragonalstructures, as a result of a partial conversion due to the TEOFtreatment. Moreover, the Gd3+ ions can give rise to anothermonoclinic phase since the pure gadolinium phosphate can occurwith the monazite structure.27 Even though the amount of thesesecondary phases is so low that it cannot be detected by XRD,their occurrence is easily noticed via Eu3+ spectroscopy due tothe high sensitivity of this technique. The excited-state kineticbehavior of this sample can be well fitted by a biexponentialdecay curve, which corroborates with the occurrence of twoEu3+ emitting centers in the (Y,Gd)PO4 host. The deconvolutionof the experimental decay curves yields lifetimes of τ1 ≈ 3.84and τ2 ≈ 0.62 ms.

The emission spectrum of the calcined (Y,Gd)PO4:Eu3+

sample displays a strong intensification of the 5D0 f7F2

transition with regard to 5D0 f7F1, which is a consequence of

the occupation of the D2d sites in the tetragonal structure. Thisfact, in addition to the narrowing of the emission bands andthe high intensity of the 5D0f

7F4 transition, leads to very highchromaticities for this compound (x ≈ 0.68, y ≈ 0.34). As forthe TEOF-treated sample, the emission profile does not dependon the excitation wavelength (on the CTB at 230 nm, on theGd3+ 6IJ level at 271 nm, or on the Eu3+ 5L6 level at 394 nm).In the case of the calcined powder, the occurrence of more thanone symmetry site for the Eu3+ ion is also evidenced by thepresence of two components centered at 579.6 and 580.0 nm,ascribed to the 5D0 f

7F0 transition. The occurrence of twoEu3+ sites can be associated, as already discussed, with a smallamount of monoclinic GdPO4 coexisting with the tetragonal(Y,Gd)PO4 phase. Nevertheless, despite the presence of twoEu3+ centers, the excited-state decay curves of this sample can

Figure 7. Emission spectra (λexc ) 394 nm) of the LaPO4:Eu3+ samples(a) after the TEOF treatment and (b) after calcination at 900 °C (inthis case, obtained at ω0 ) 50).


always be well fitted by monoexponential functions, regardlessof the monitored wavelengths (λem ) 589.0 ( 0.5, 591.0 (0.5, 595 ( 10, or 612 ( 10 nm) or the excitation mechanism(6IJ or 5L6) (Supporting Information). This characterizes pseudo-first-order excited-state kinetics, which can be the result of anefficient energy transfer mechanism from one site to another(thus reducing the lifetime of one of the emitting centersconsiderably) or of the fact that the two centers have quitesimilar luminescence lifetimes. The second explanation seemsto be more appropriate since monitoring of the individualcomponents of the 5D0 f

7F1 transition with 0.5 nm resolutionyields similar lifetimes (Supporting Information).

For better evaluation of the Eu3+ luminescent behavior inthe synthesized phosphate hosts, the radiative (ARAD) andnonradiative (ANRAD) decay rates, as well the excited-statequantum efficiencies, were calculated from the emission spectraand luminescence lifetimes.32,33 The emission quantum efficiencyof a determined excited state is defined as the ratio betweenthe rate of radiative deactivation and the total rate of deactiva-tion. Considering the Einstein spontaneous emission coefficients(ARAD), the emission quantum efficiency is given by

The total rate of excited-state deactivation (ARAD + ANRAD) isequal to the reciprocal of the mean excited-state lifetime:

Therefore, the quantum efficiency can be written as

Taking the emission of the Eu3+5D0 level, the radiative rate ofexcited-state deactivation can be considered as a sum of theparticular rates for each of the 5D0f

7FJ (J ) 0-6) transitions:

For being allowed through a magnetic dipole mechanism, the5D0 f

7F1 transition has a calculable rate of radiative deactiva-

TABLE 1: Luminescence Lifetimes, Radiative and Nonradiative Decay Rates, Quantum Efficiencies, and ChromaticityCoordinates29 Obtained for the Eu3+-Doped Phosphates under 394 nm Excitationa

lifetime (ms) chromaticity

τ1 τ2 ARAD (s-1) ANRAD (s-1) η (%) x y

LaPO4:Eu3+ (TEOF-treated)ω0 ) 15 3.90 0.79 233 142 62 0.637 0.360ω0 ) 30 3.85 0.75 231 143 62 0.636 0.362ω0 ) 40 3.87 0.90 234 144 62 0.639 0.358ω0 ) 50 3.68 0.70 226 145 61 0.638 0.359

LaPO4:Eu3+ (calcined)ω0 ) 15 4.76 199 10 95 0.664 0.334ω0 ) 30 5.04 192 6 97 0.670 0.328ω0 ) 40 4.53 203 18 92 0.667 0.332ω0 ) 50 5.56 178 2 99 0.672 0.327

(Y,Gd)PO4:Eu3+ (TEOF-treated) 3.84 0.62 151 280 35 0.668 0.331(Y,Gd)PO4:Eu3+ (calcined) 3.72 224 99 69 0.676 0.323

a Luminescence lifetimes are accurate within 5%; the calculated parameters are accurate within 10%.

Figure 8. Excitation spectra (λem ) 590 nm) of the prepared(Y,Gd)PO4:Eu3+ nanopowder (a) after the TEOF treatment and (b) aftercalcination at 900 °C (the asterisks indicate Gd3+ excitation lines).

Figure 9. Emission spectra (λexc ) 271 nm) of the (a) hexagonal (afterthe TEOF treatment) and (b) tetragonal (after calcination at 900 °C)(Y,Gd)PO4:Eu3+ sample. Insets: amplification of the 5D0f

7F0 transitionranges.

η )ARAD

ARAD + ANRAD(1)

ARAD + ANRAD ) τ-1 (2)

η ) ARADτ (3)

ARAD ) ∑J

A0-J (4)


tion and can be used as a reference for the determination of theother A0-J rates. The value of A0-1 can be calculated from

where n is the refractive index of the material and σ0-1 is theenergy baricenter of the 7F1 level (calculated from the emissionspectra). The refractive index considered for our calculationsis 1.75 since the tabulated values found for the monazite andxenotime vary between 1.70 and 1.80. The other A0-J radiativerates are calculated from

where σ0-J is the energy baricenter of the 7FJ level and S0-J isthe area corresponding to the 5D0f

7FJ transition in the emissionspectrum. Therefore, by applying eqs 3 and 4, the emissionquantum efficiencies are calculated from the emission spectra.In the cases where two distinct Eu3+ emitting centers exist, amean lifetime obtained by fitting the experimental curves formonoexponential decays was utilized. The TEOF-treated LaPO4:Eu3+ samples display relatively high quantum yields, around60%, thus attesting that, despite the relatively low temperatureused during the drying procedure, efficient red phosphors areobtained. In these cases, the most prominent contribution forthe nonradiative decay rate arises from the low crystallinity ofthe samples, which provides multiphonon pathways for excited-state deactivation. In the (Y,Gd)PO4:Eu3+ phosphor, this effect

is more pronounced due to the probable coexistence of manyphosphate phases, thus resulting in a higher ANRAD rate. Afterthe calcination, this effect is largely reduced, resulting in a∼70% quantum efficiency. Nevertheless, the more interestingresult is observed for the calcined LaPO4:Eu3+ phosphors, which,for having a very long lifetime, possess quantum yields closeto unity, as a result of nonradiative decay rates close to zero. Inthis way, the synthetic approach allows one to obtain veryefficient Eu3+-doped phosphors whose high crystallinity andnanostructural character provide a means for a very long livedluminescence with a quite high quantum efficiency.

3.3.2. Blue-Emitting Phosphors. The proposed route wasapplied for the synthesis of tetragonal and monoclinic blue-emitting Tm3+-doped phosphates. The excitation spectra ofLaPO4:Tm3+ and YPO4:Tm3+ (Figures 10a and 11a) are, in bothcases, composed by a broad band at energies higher than 33 500cm-1 (as a result of the O2- f Tm3+ charge transfer) and bythe main Tm3+ f-f absorption at ∼360 nm, ascribed to the 1D2

r 3H6 transition. In the YPO4:Tm3+ sample, this transitionoccurs as a sharper band due to the lower unfolding of the 2S+1LJ

levels in the D2d sites of the tetragonal structure; besides, inthis case the absorptions to the higher Tm3+ levels (3PJ r

3H6)can also be detected. The emission spectra of the Tm3+-dopedphosphates under 1D2 excitation (Figures 10b and 11b) aredominated by the blue 1D2f

3H4 emission at 450 nm, and theirluminescence is visible to the eye (despite its low brightness),appearing as blue. This results in high blue color purities forthese compounds (x ) 0.17 and y ) 0.07 for LaPO4:Tm3+ andx ) 0.20 and y ) 0.09 for YPO4:Tm3+), which are acceptablefor applications as blue phosphors according to the NTSCstandards (x ) 0.14, y ) 0.08).4 The other transitions within

Figure 10. (a) Excitation (λem ) 450 nm) and (b) emission (λexc )358 nm) of the LaPO4:Tm3+ sample (calcined at 900 °C). Figure 11. (a) Excitation (λem ) 450 nm) and (b) emission (λexc )

360 nm) of the YPO4:Tm3+ sample (calcined at 900 °C).

A0-1 ) 0.31 × 10-11n3σ0-13 (5)

A0-J ) A0-1

S0-Jσ0-1

S0-1σ0-J(6)


the Tm3+ f12 configuration result in low-intensity emissions inthe visible and NIR ranges, and the integration of the LaPO4:Tm3+ emissions results in an almost monochromatic color, asseen in the chromaticity diagram (Figure S24, SupportingInformation). Although it presents narrower bands, the YPO4:Tm3+ sample has the lower color purity due to the slightly largerintensity of the other visible emissions arising from the 1D2,1G4, and 3F2,4 levels. Nevertheless, although they present a lowluminescence yield due to the competitive infrared emissionswithin the complicated Tm3+ energy levels, the high chemicalstability and high emission color purity of these materials makethem suitable for applications under VUV excitation.

3.3.3. UpconWersion Luminescence of YbPO4:Er3+. Figure12a shows the upconversion (UC) luminescence spectrum ofthe YbPO4:Er3+ sample under 980 nm laser excitation. Ameasurable luminescence appears only after the thermal treat-ment at 900 °C since a higher crystallinity is achieved only inthis way, thus avoiding the nonradiative deactivation of theintermediary 4I11/2 and the emitting 2H11/2, 4S3/2, and 4F9/2 levelsin the Yb3+/Er3+ couple. Although the pure tetragonal YbPO4

is known to display a cooperative UC luminescence,3 the UCefficiency of the Yb3+/Er3 couple in this host is expected to bevery low, because the higher phonon energy in this materialtakes place at ∼1100 cm-1. This leads to very strong vibronicinteractions of the involved ions with their surroundings, thusdiminishing the lifetime of the intermediary levels and reducingthe global efficiency of the phosphor. Nevertheless, in this case,this is overcome by the high concentration of the sensitizer Yb3+

ions, which leads to a strong absorption at 980 nm as a resultof the spin-allowed 2F5/2r

2F7/2 transition, followed by the Yb3+

f Er3+ energy transfer, which in turn gives rise to the 4I11/2

excited state of Er3+. Another NIR photon is then transferredfrom the Yb3+ ions, thereby leading to the population of theEr3+4F7/2 level and resulting in its green (2H11/2, 4S3/2 f

4I15/2)and red (4F9/2 f

4I15/2) emissions. In this case, the spectrum isdominated by the spin-allowed 4F9/2f

4I15/2 Er3+ transition, thusresulting in a predominantly red UC luminescence.

The variation of the UC luminescence intensity (computedas the integrated area of the monitored transition) as a functionof the laser power (Figure 12b) results, as expected, in a linearbehavior in the log(intensity) against log(power) plot, withslopes of 2.4 and 1.8 for the 2H11/2 f

4I15/2 and 4S3/2 f4I15/2

transitions, respectively. Within the experimental error, thesevalues corroborate with a two-photon excitation scheme for theUC process in this phosphate. On the other hand, due to itshigh intensity, the 4F9/2 f

4I15/2 transition displays an indepen-

dence of the laser power, indicating that an NIR photonexcitation saturation occurs even at 500 mW. This is a resultof a very efficient nonradiative decay from the populated 4F7/2

level to the emitting 4F9/2 level (probably due to interactionswith lattice phonons and defects), which makes 4F9/2 f

4I15/2

the most prominent transition in the UC process, so that itspower dependency can only be measured at lower laser powers.

4. Conclusion

In summary, a versatile solution route has been evaluatedfor the preparation of a wide range of nanosized rare-earthorthophosphates. The synthetic approach, based on the nanore-actor concept, involves the dispersion of the reactive precursorsin the water pools of reverse microemulsions (using CTAB/1-butanol as the surfactant mixture and n-heptane as the continuousphase). The procedures lead to the production of elongatednanoparticles or nanowires of uniform thickness, whose lengthand degree of aggregation can be controlled by varying thewater:surfactant ratio or by the addition of a complexing agentfor the RE3+ ions (e.g., citric acid). This growth process isrelated to the microemulsion dynamics, in which the rate ofprecipitation is higher than the rate of micellar dissociation, thusfavoring the elongation of the particle seeds. As expected, thecomplexing action of H3cit retards the inorganic precipitation,thus stabilizing the formed nanorods and diminishing theirdegree of aggregation.

The procedure favors the formation of hydrated hexagonalphosphates for both the lighter and heavier lanthanoids, withvisible effects of the structure-directing action of the micro-emulsion systems on the powder XRD patterns. The analysescarried out herein confirm the purity of the synthesized materials,whose degree of hydration is also dependent on the observedmorphology. Despite the low temperatures employed here, theTEOF treatment has been shown to efficiently eliminatehydration waters, resulting in Eu3+-doped phosphors with arelatively high quantum yield. Nevertheless, after the calcina-tions at 900 °C, these phosphates present even higher lumines-cence intensities and quantum efficiencies close to unity.Moreover, the synthesized LaPO4:Tm3+ and YPO4:Tm3+ samplesefficiently display blue emissions with high color purity, andthe YbPO4:Er3+ phosphor presents intense red upconversionunder NIR excitation. In conclusion, the proposed syntheticmethod is very suitable for the production of several nanosizedRE-containing luminescent phosphates, whose composition andmorphology can be designed for a wide range of applications.

Figure 12. (a) Upconversion luminescence spectrum of YbPO4:Er3+ under 980 nm laser excitation (500 mW). (b) Laser power dependence of theupconversion intensity measured at the 2H11/2 f

4I15/2 (λ ) 525 mn), 4S3/2 f4I15/2 (λ ) 545 mn), and 4F9/2 f

4I15/2 (λ ) 650 nm) transitions.


Acknowledgment. We thank the Brazilian agencies CAPES,CNPq (Proc. 480003/2009-2, O.A.S.), FAPESP (Proc. 2008/09266-5, P.C.d.S.F.), RENAMI, and inct-INAMI for financialsupport. We are also grateful to Professors E. Tfouni, M. E. D.Zaniquelli, S. J. L. Ribeiro, and J. M. A. Caiut for utilizationof the instrumental facilities and to Drs. C. M. C. P. Mansoand R. F. Silva for helpful discussions.

Supporting Information Available: Thermal analysis curves,FTIR spectra of the obtained powders, microemulsion dynamiclight scattering and nanoparticle �-potential determination data,additional X-ray diffractograms, additional EDS spectra andSEM-EDS mapping images, chromaticity diagrams, additionalluminescence spectra, and decay curves for luminescencelifetime determination. This material is available free of chargevia the Internet at http://pubs.acs.org.

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